Handbook of Aggregation-Induced Emission, Volume 1. Группа авторов

the absorption spectrum of cis‐13 in CH₂Cl₂ had a gradual absorbance increase at 355 and 278 nm but a constant decrease at 315 nm with irradiation time, and showed an obvious isosbestic point at 336 and 302 nm, indicating the conversion from cis‐isomer to trans‐one. In CDCl₃, after irradiation by a 365‐nm portable UV lamp for one hour, another set of signals appeared beside signals of cis‐13 in the ¹H NMR spectrum. Especially, besides the signals of the aromatic proton near to the oxygen atom (6.71 ppm, double), benzyl methylene (4.82 ppm, single), and the ethylene proton close to the aromatic ring (4.09 ppm, triple), new peaks that were well separated and had the same shape and split with that of cis‐isomer appeared at a lower field. The integral area of these new peaks was all almost equal to that of cis‐13, suggesting a 50% conversion of cis‐isomer to trans‐one. In DMSO, the converted trans‐isomer (about 20%) by light could completely come back to cis‐isomer under heating at 180 °C. This result suggested that the double bond of the TPE derivatives was easy to rotate in the excited state.

Figure 3.22 Schematic diagram of the binding of TPE cycle diammoniums 11 (a) and 12 (b) to the DNA strand.

      The TPE ammonium 13 was tested for the detection of fish sperm DNA. It was found that cis‐13 had a high sensitivity (139 pM) and high‐intensity ratio I_max/I₀ (6.6), whereas a mixture of cis/trans‐13 about 1 : 1 displayed a low‐sensitivity (326 pM) and low‐intensity ratio I_max/I₀ (4.5), indicating that the cis‐isomer was a much better DNA sensor than the trans‐one due to more restriction of intramolecular motion including the double bond rotation (see Figure 3.23).

There are more examples in which TPE cycle at the cis‐position could emit fluorescence in solution due to the restriction of double bond rotation. Rathore et al. synthesized a class of stilbenoprismands 14 via an intramolecular McMurry coupling reaction, which contained a TPE core‐bearing cycle at the cis‐side (see Figure 3.24 left) [46]. While the parent TPE showed no detectable emission, the solution of 14 showed significant emission under similar conditions although there were two substituted phenyl rings that could freely rotate. By the direct coordination of TPE tetraimidazolium salts with silver or gold, Hahn and Strassert [47] prepared TPE dicycles 15 on the cis‐side. The TPE dicycles 15 showed strong emission in solution but TPE tetraimidazolium salts emit no fluorescence, demonstrating the important role of the RDBR process on the fluorescence (see Figure 3.24 right).

Hu et al. [48] reported one TPE derivative dicycle 17, in which two pairs of the phenyl rings at the cis‐position were connected by two diacetylenes. While the no cyclized TPE reactant 16 emitted very weak fluorescence in solution, the emission of 17 is very strong even in dilute solution. This fluorescence enhancement after cyclization at the cis‐position should mainly come from the RDBR process (see Figure 3.25).

Figure 3.23 (a) The cis‐/trans‐isomerization of 13 under a 365‐nm light irradiation and heating. (b) Change in UV–vis spectrum of compounds cis‐13 in dichloromethane under an irradiation of 365‐nm light from fluorophotometer for different periods. [cis‐13] = 1.0 × 10⁻⁵ M. (c) The ¹H NMR spectra of cis‐13 in CDCl₃ before and after irradiation by a 365‐nm portable UV lamp for one hour.

Figure 3.24 The structure of AIEgens 14 and 15.

Figure 3.25 The synthesis of the emissive molecule 17.

      3.2.4 Research of Theoretical Calculation on RDBR

      The effect of restriction of the double bond rotation can be shown intuitively from the experimental phenomenon, but to understand in more details the behavior of the double bond in the excited state and the role it plays, more theoretical studies are needed. In this regard, quantum‐computational simulation and ultrafast time‐resolved spectroscopy are two major methods. The former can simulate the changes in molecular energy and structure during the fluorescence process, by comparing the energy barriers of different decay routes in the excited state to find where the nonradiative relaxation takes place. The latter can probe and resolve the excited‐state dynamics and reaction processes by monitoring the structural changes and the emergence of new species, finding nonradiative process [49, 50].

Through computational studies, detailed activities of the molecule in the excited state can be described, especially in single‐molecular behaviors, which is similar to the state of AIEgens in solution. In this part, an emerging theory called the restricted access to conical intersection (RACI) mechanism has been used for explaining the AIE effect of many AIEgens. Some conical intersections (CIs) are blamed for the weak emission of AIEgens in solution, such as π twist, photocyclization, ring puckering, excited‐state intramolecular proton transfer (ESIPT), bond stretch, and so on [51–53]. Recent computational studies have contributed to the clarification of the excited‐state decay process of TPE in the solution state, and the results indicated the presence of a CI in the ultrafast quenching process of TPE including photocyclization and/or π twist, rather than the propeller‐like rotation of the side phenyl groups. Despite an ongoing debate, there are reports revealing that the twist of the double bond is a typical CI for the deactivation of TPE. Excitation to the S₁ state (HOMO → LUMO) causes an elongation of the double bond, which initiates the twisting dynamics. This motion stabilizes the first excited state (S₁) and destabilizes the ground state (S₀), ultimately causing the two states to become degenerate, which are referred to as CI (see Figure 3.26). The dynamic process explains subsequent E–Z photoisomerization and weak emission of isolated TPE molecules.

Figure 3.26 A brief illustration of the conical intersection